Various ways are recognized in the photographic industry for releasing a PUG from a compound, such as a coupler, in a photographic material and process. Release can be direct, for example upon reaction of the coupler with oxidized color developing agent during processing, or it can be indirect through a linking or timing group. Linking and timing groups provide the ability to control the timing and rate of release of a PUG in a photographic element, as well as the rate and distance of diffusion of the PUG in the element during processing.
U.S. Pat. No. 4,248,962 describes compounds that release a PUG, such as a development inhibitor group, through a timing group which functions by (intramolecular) nucleophilic displacement reaction. Other examples of compounds that are capable of releasing a PUG are described in U.S. Pat. Nos. 4,409,323 and 4,861,701. In U.S. Pat. No. 4,409,323, compounds are described which release a PUG by a mechanism which involves electron transfer down a conjugated chain. In U.S. Pat. No. 4,861,701, sequences of timing groups are utilized to release a PUG and to provide desirable control over the impact of the PUG on photographic properties.
European Patent Applications 0 499 279 and 0 438 129 describe photographic couplers and compounds which allegedly provide superior release rates over compounds described in the aforementioned U.S. Pat. No. 4,861,701. Specifically, these applications describe photographic compounds having a heterocyclic timing nucleus (typically pyrazole) attached to a coupler moiety through an --O--C(O)-- or --OCH.sub.2 -- group, or other group capable of releasing the heterocyclic timing nucleus by electron transfer down an unconjugated chain. However, these compounds, especially those described in European Patent Application 0 499 279, do not provide for any flexibility in their rate of release of a PUG, or in their synthetic design, as they are limited by the presence of the particular first timing or linking group. More importantly, though, the compounds in both of these applications exhibit poor stability and decompose when stored for prolonged periods under tropical conditions. Thus, they are of limited practical value in today's photographic industry.
A need has therefore existed for a photographic coupler that is synthetically simple to manufacture; that is capable of providing a wide range of release rates depending upon the particular selection of timing or linking groups and the substituents thereon; and that is stable when stored for prolonged periods, especially under tropical conditions. The coupler which is needed should be capable of releasing a PUG, such as a development inhibitor, thereby providing interlayer interimage effects and increased acutance for the image produced upon processing photographic material containing the coupler.